The related art will now be described, using a slewing drive apparatus for a shovel as an example.
A shovel includes a crawler type lower propelling body, an upper slewing body installed on the lower propelling body rotatably around a shaft vertical to the ground, and a work attachment attached to the upper slewing body.
Here, a hybrid shovel or an electric shovel includes a slewing drive apparatus for causing the upper slewing body to slew. The slewing drive apparatus includes an electric motor (also referred to simply as a motor hereafter) serving as a drive source, and a speed reducer that reduces speed of a rotation force of the motor and transmits the reduced rotation force to the upper slewing body serving as a driven portion.
The motor includes a motor shaft, and the speed reducer includes a speed reducer output shaft connected to the motor shaft. The motor and the speed reducer are provided in a row in an axial direction of the slewing drive apparatus (to be referred to hereafter as a apparatus-axis direction) such that both center axes (the motor shaft and the speed reducer output shaft) are located along the same line. Further, the motor and the speed reducer are mounted to an upper frame in such a vertical arrangement that the motor is located at a top.
The speed reducer includes a casing surrounding a speed reducer chamber, and a speed reduction mechanism that is provided in the speed reducer chamber and includes at least single-stage planetary gear mechanism, for example. The planetary gear mechanism includes a sun gear, a planetary gear, and a ring gear. An output of the speed reducer is transmitted to the upper slewing body via a pinion provided on the speed reducer output shaft and a slewing gear provided on a lower frame of the lower propelling body.
Lubricating oil for lubricating the speed reduction mechanism is injected into the speed reducer chamber (the casing). When the drive apparatus is operative, a temperature of the lubricating oil increases, and as a result of this temperature increase, a volume of the lubricating oil expands. Accordingly, an oil level of the lubricating oil rises, and as a result of the rising oil level, the lubricating oil may overflow. Further, when an internal pressure of the speed reducer chamber increases, a structure for sealing the motor shaft that is introduced into the speed reducer chamber may be damaged.
Hence, in a conventional technique (see Patent Documents 1 and 2, for example) for suppressing overflow of the lubricating oil and damage to the structure for sealing the motor shaft, an air chamber provided in a connecting part between the motor and the speed reducer is maintained at a pressure corresponding to atmospheric pressure (this function will be referred to hereafter as an air breather function). More specifically, in this technique, a part (to be referred to hereafter as an air breather) that enables the air chamber to communicate with the outside is provided to maintain the interior of the air chamber at a pressure corresponding to atmospheric pressure.
Meanwhile, a nacelle slewing drive apparatus for a wind power generation facility described in Patent Document 3 includes a clutch and a brake provided between a motor and a speed reducer, and an air chamber and an air breather disposed on an outer periphery of the clutch and brake.
The technique of providing a dedicated air chamber having an air breather function, as described in Patent Documents 1 and 2, is basically achieved by adding the air chamber between the motor and the speed reducer, and therefore an overall length (overall height) dimension of the device increases. As a result, a large space is required to install the drive apparatus.
It is therefore particularly difficult to dispose the drive apparatus described in Patent Documents 1 and 2 in a construction machine such as a shovel, in which space for installing the drive apparatus is limited in both a height direction and a horizontal direction. More specifically, a torque per unit size of an electric motor serving as a slewing drive source is smaller than that of a hydraulic motor. Therefore, an electric motor used to obtain an equivalent torque to that of a hydraulic motor is larger than a hydraulic motor, and as a result, the space for installing the drive apparatus is limited in the height direction and the horizontal direction.
Since the height dimension of the air chamber in an actual device is limited, it is impossible to secure sufficient volume in the air chamber. As a result, the air breather function may not be realized sufficiently.
In the technique described in Patent Document 1, the air chamber is connected to a buffer tank disposed on the outside of the speed reducer by a passage, and therefore an internal volume of the buffer tank is used as a part of the air chamber.
However, the buffer tank is disposed on the outside of the speed reducer under the premise that since the oil level of the lubricating oil takes the funnel-shape during an operation, the lubricating oil will flow into the buffer tank. During an operation on sloping ground, therefore, the lubricating oil likewise flows into the buffer tank due to the tilt of the construction machine. When, in this condition, the oil temperature is increased by direct sunlight or the like, air in the buffer tank expands, and as a result, the lubricating oil may overflow.
In other words, the air breather function is not realized sufficiently even with a configuration employing a buffer tank.
Furthermore, since the buffer tank is disposed on the outside of the speed reducer, a diameter dimension of the drive apparatus increases. As a result, a large space is required to install the drive apparatus in a construction machine.
In the technique described in Patent Document 3, meanwhile, the air chamber is formed on the outer periphery of the clutch and brake. Therefore, when an air chamber having a large enough volume to realize a sufficient air breather function is formed, height direction and diametrical direction dimensions of the drive apparatus increase.
In other words, although the technique described in Patent Document 3 can be put to practical use in a wind power generation facility having substantially no dimensional limitations, the technique cannot easily be applied to a construction machine having severe dimensional limitations, such as a shovel.
Patent Document 1: Japanese Patent Application Publication No. 2008-232270
Patent Document 2: Japanese Patent Application Publication No. 2002-357260
Patent Document 3: Japanese Patent Application Publication No. 2004-232500